It is a common problem in isolated converters to provide a proper bias for both primary and output circuitry, particularly during start-up or restart of the converter. Usually a controller (pulse width modulated (PWM) is one example) is on the input side and the feedback signal is provided via an opto-coupler, while synchronous rectifiers are self-driven from the transformer windings. There are two drawbacks in using this approach. First, the use of an opto-coupler generally limits the bandwidth of the regulation loop and the maximum ambient temperature and temperature of the printed circuit board (PCB) to less than about 85° C. Secondly, the self-driven synchronous approach is generally not a good solution for higher frequencies.
In addition, protection such as over-voltage protection (OVP) has to be on the output side, and might require an additional opto-isolator just for over-voltage protection. Therefore, there is an advantage to having the control circuit on the output side. One problem is to provide a needed initial bias voltage across the isolation break, before the converter is started. One possible solution is to have a separate isolated converter that will provide the bias voltage. Such a solution would require an additional magnetic core and, if realized employing planar magnetics, would consume a lot of board space.
One viable solution for providing initial bias and enable signals was described in U.S. Pat. No. 6,724,642, Method and apparatus for providing an initial bias and enable signal for a power converter, issued to the same inventor as the instant application. The '642 patent is incorporated herein by reference in its entirety. The '642 patent uses an isolated coreless transformer having windings formed in or on layers of a printed circuit board (PCB). As a coreless solution, the bias circuit of the '642 patent does not require any additional magnetic material, such as is used in conventional planar magnetic solutions. However, the windings of a coreless transformer occupy precious space on the layers of the power converter PCB, as well as limit the placement of components in the vicinity of the windings.
What is needed is a solution to the problem of how to supply bias power across an isolation barrier that uses less area on the PCB.